The need for higher performance materials in the automotive industry has increased more than ever in past decades Improved technology has produced cars that not only drive faster and more economical but also have more electronic options such as in multimedia devices. Once where an auxiliary-in port was rare in an automobile, today they are fully connected with USB and Bluetooth ports and the like that integrate media players and cell phones directly into factory sound systems. With this increase demand on automobile sound systems, a requirement for better acoustics in the automobile has been seen in recent years.
Acoustic impedance is an important physical property in substances that determine the substances ability to absorb sound. Specific Acoustic Impedance is the ratio between the sound pressure and the particle velocity produced by a sound wave moving through the substance. When sound waves pass through any physical substance, pressure of the sound waves causes the particles of the substance to move. Specific Acoustic Impedance is also directly related to the resistance of airflow. Specific Acoustic Impedance is measured in RAYLS. The higher number of RAYLS, the lower the velocity of sound transmission through a medium. A fabric is considered to have better acoustic sound absorption with a higher RAYLS number.
Having external noises removed from the automobile cabin is an issue that automobiles of the past did not need to contend with, however, with advancements in digital media and sound systems for cell phones, music and the like, the importance of filtering out external sounds from the driver's cabin is more important than ever before.
In the past, wind noise was a significant cause of noise in automobiles. When automobiles were not aerodynamically shaped as they are now, various objects like chrome molding surrounding the windshield glass created turbulence resulting in wind noise. Wind turbulence around door mounted minors was also a notorious noise maker. However, today's streamlined designs have significantly reduced such turbulence along with its accompanying noise. However, even with the aerodynamic design, today's cars still have air rushing past the vehicle creating noise heard through the doors.
Today noise created by the road traveled is one of the largest contributors to noisy automobile interiors. Road noise typically originates from tires running over road surfaces and may take on two forms of noise called acoustic noise and conductive noise.
Acoustic noise is transmitted from the surface of the road through the air and into the driver's cabin. This noise may be treated by adding acoustical material to surround the driver's cabin and doors. However such materials lack the structural properties and strength as well as the light weight needed for automotive applications.
Conductive road noise is caused by vibrations conducted from the road surface through the automobile's tires and suspension into the driver's cabin. Even with advancements in tire technology to achieve quieter tires noise is still produced by the road and enters into the driver's cabin. With today's common use of run flat tires or low profile tires, these noises are compounded due to the significant amount of less rubber on the tires of today's automobiles. Thus there still remains a need for a material that is structural sound for automotive applications lightweight and produced the acoustic effect necessary to eliminate noise in the driver's cabin for today's electronics and sound systems.
In addition, due to the higher performance automobiles, generation of heat from the engine has become an ever increasing problem. Within the Automotive Industry over the past 50 years, there have been increased requirements for improved flame resistance. After many car fires, the industry adopted standard MVSS 302 from the National Highway Traffic Safety administration. Even with the evolution of all-electric vehicles, there is a need for improved fire resistant and low-smoke fabrics. This test is a horizontal burn test that tests among other things the flammability of the interior of the automobile and engine compart. The test involves taking a sample 14″×4″ and placing the sample on a metal frame. A flame via a Bunsen burner or other device is applied under the front edge of the 4″ width. The flame is placed under the fabric and the fabric is allowed to burn for 15 seconds.
Originally this flame test required that fabric burn less than 101.6 mm/minute (4 inches/minute). The presence of any noxious gases is also observed during the test. Over the years, many automobile manufacturers have made the flame test results more restrictive. Some automotive manufacturers have reduced the burn rate to 2 inches per minute and down to 1 inch per minute. The requirements for some applications, for example such as engine compartment areas, have required that the sample be SE (self-extinguishing) with a zero burn rate and in some cases with the strictest requirement which is DNI (did not ignite).
Recently, several Automotive Manufacturers have focused on another flame test developed by Underwriters Laboratories, a flame test UL-94, which is a vertical flame test. This flame test requires a 5″×½″ fabric sample held vertically. A flame is placed under the fabric sample for two-10 second intervals.
The results of the flame test UL-94 are report as:                HB: slow burning on a horizontal specimen; burning rate <76 mm/min for thickness <3 mm or burning stops before 100 mm        V-2 burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.        V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.        V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.        5VB: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole.        5VA: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole.        
The automotive industry, among other industries, have found it very difficult if not impossible to find fibrous nonwoven fabrics that can meet both the above acoustic and flame resistance requirements. Fabrics are needed in the automotive industry for good acoustical qualities and these fabrics must be located near the engine or exhaust system so flame resistance properties are essential. There is also a need for fabrics with low-smoke properties. Further, there is a need for fabrics that are moldable with standard thermoplastic molding equipment, yet still have excellent thermal stability after molding. There still remains in the art a need for a moldable nonwoven fabric with enhanced flame resistance and excellent thermal stability.
Single layer nonwovens have tried to increase RAYLS by using more fine fibers to create a denser medium to reduce air flow, and hence reduce sound transmission. However this technique has not been achieved at a practical cost or weight.
At the same time, the Automobile Manufacturers have found a need for an Underbody Shield to be moldable, durable and fit under the vehicle to prevent road and wind noise from penetrating upward into the passenger compartment. Further, it has been shown that these composites are weighing close to 2,000 gsm (grams per square meter) to achieve the noise reduction levels desired. This amount of weight is too much for an automobile part due to stability of the vehicle, drag and energy efficiency concerns.
In addition, due to the high heat exposure from engine parts, a need exists for a product that does not exhibit failure during heat aging up to 150° C.; has resistance to water, oil, and engine fluids, has low flame spread and low smoke, and is recyclable back into itself. Further, these moldable products must have excellent abrasion resistance against sand & gravel.
Further environmental factors for automotive products include the exposure to moisture. Any materials coated with heat resistant coatings or coatings that increase acoustic impedance are easily worn away by the harsh environmental conditions of the undercarriage and wheel wells of the automobile. Rain, snow, ice and salt as well as other particles are common environmental conditions that affect an automobile's undercarriage and wheel wells as well as other portions of the automobile. Any coatings or non-structural material used that contain heat resistant or acoustical properties are easily worn away in such environments.
Thus there still exists a need for an acoustic absorber that is thermo-formable or otherwise moldable, light weight, resistant to water penetration and other environmental factors, flame resistance, and has a high RAYLS number.